Exchange 2010 not accepting external messages

All, I am having a problem between two of my exchange 2010 R2 servers. They are at different sites. When server 1 tries to send external email to server 2, the following error is in the queue:

451 4.4.0 Primary target IP address responded with:" 421 4.7.0 too many errors on this connection, closing transmision channel." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failover.

My first research pointed me something with the queue database, but we have 25 gigs free on the Server 2.

This just started last night, and I have yet to find a resolution. Any help would be appreciated.

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